1. Field of the Invention
This invention relates to multi-layer polymer compositions that comprise two outer layers and an adhesive tie layer. This invention specifically relates to a multi-layer polymer composition comprising a polar polymer, e.g. polyvinylchloride (PVC), as the first outer layer, a non-polar polymer, e.g., polypropylene, as the second outer layer, and a unique blend of polymers for the adhesive tie layers that allows the composition to be easily and advantageously coextruded.
2. Description of Related Art
It is often desirable to adhere polar polymers to non-polar polymers. Non-polar polyolefins, such as polyethylene and polypropylene, have traditionally been available at a lower cost than polar polymers. Using the non-polar polymer for the bulk of a fabricated part, with only a thin layer of polar polymer on the surface, can provide attributes of the polar polymer at an overall lower cost.
As a specific example, PVC siding used in the housing construction industry typically has poor dimensional stability. Because of its low glass transition temperature, Tg, and flexibility, it can expand and contract with swings in atmospheric temperature, which can be a problem when used as housing siding. To minimize this problem, only heat-reflecting, light colors can be used.
To overcome the problem of poor dimensional stability, the PVC can be combined with a thick substrate of polypropylene, and/or glass fiber-reinforced or filled polypropylene, which are stiffer at all temperatures and more dimensionally stable. This can reduce cost, also, since the more expensive PVC can be a much thinner layer than that used in conventional siding. With the dimensional stability problem solved, darker color PVC's can also be used.
The problem remains, however, of adhering the polar polymer, e.g., PVC, to the non-polar polymer, e.g., polypropylene, in a structurally sound and cost-efficient manner.
Attempts have been made to solve this problem in the past. For example, U.S. Pat. No. 6,045,732 (Nakatsuji et al.), discloses a multi-layer molded article having a skin material layer of a vinyl chloride resin and a propylene resin core. They disclose an adhesive layer (A), used to combine the vinyl chloride resin and propylene resin core, containing two layers, one layer (polyester resin and epoxy-group containing ethylene copolymer) bonded to the vinyl chloride resin, one layer (at least one copolymer selected from an ethylene copolymer and epoxy group-containing copolymer) bonded to the propylene resin core. Alternatively, adhesive layer (B), comprising an ethylene resin composition containing a polybasic carboxylic acid and an epoxy group-containing copolymer, can be used. These multi-layer articles, however, must be formed by molding under heat and pressure to form the final multi-layer article. The adhesive layers are formed in advance in a separate step by, for adhesive layer (A), co-extruding the adhesive layer, or by powder molding, inflation molding or extrusion, for adhesive layer (B).